Section 4-3. Ethernet

4-3. Ethernet

• Autonegotiation of link speed and duplex mode for 10/100/1000BASE-T is possible through the functions standardized in IEEE 802.3u and 802.3ab. The two endpoints of a connection exchange capability information and choose the highest common speed and duplex supported by both.

• Ethernet ports are referenced by module and port numbers (mod/port) on COS switches and by interface type and number (interface and one of ethernet, fastethernet, gigabitethernet, or tengigabitethernet) on IOS switches.

• If certain problems are detected on a port, the switch automatically moves that port into the errDisable or "error disabled" state. This minimizes the effect that the problem port could have on the rest of the network.

• Ports in errDisable can be automatically reenabled or recovered after a timeout period, or they can be manually recovered. In either case, determine and correct the problem condition before attempting to recover errDisable ports.

Configuration

1.

(Optional) Assign a descriptive name to the port:

COS	set port name mod/port [port-name]
IOS	(interface) description port-name

The description port-name (text string) is assigned to the port for human use. Usually the description includes a reference to the location, function, or user of the port.

2.

(Optional) Set the port speed:

COS	set port speed mod/port {10 |)100 |)1000 | auto}
IOS	[View full width] (interface) speed {10 | 100 | 1000 | auto | nonegotiate}

You can set the port speed to one of: 10 (10 mbps for 10, 10/100, and 10/100/1000BASE-T ports), 100 (100 mbps for 10/100 and 10/100/1000BASE-T ports), 1000 (1000 mbps for 10/100/1000BASE-T ports), auto (autonegotiate the speed for 10/100 and 10/100/1000BASE-T ports; the default), or nonegotiate (don't autonegotiate the speed; IOS only). The speeds of 10BASE-T, 100FX, and Gigabit Interface Converter (GBIC) ports are fixed and cannot be set with this command.

TIP

Choosing the auto speed for a port (the default) enables the port to participate in a negotiation with the far end of the link. The two endpoints exchange information about their capabilities and choose the best speed and duplex mode supported by both. If one endpoint of a link has autonegotiation disabled, however, the other endpoint can only sense the link speed from the electrical signals. The duplex mode can't be determined and is left to the current default mode.

If you need to set the speed and duplex mode of the switch port to something other than auto, be sure to set the device at the far end of the link to the same values.

Generally, if a 10/100/1000BASE-T switch port is connected to another similar switch port or to a mission-critical device such as a server, router, or firewall, it is best to set the speed and duplex to a fixed value. By so doing, you eliminate any possibility of autonegotiation forcing the port to a lower speed in the future.

3.

(Optional) Set the port duplex mode:

COS	set port duplex mod/port {full |)half |)auto}
IOS	(interface) duplex {full |)half | auto}

You can set the duplex mode to one of full, half (IOS default), or auto (autonegotiate the duplex mode; COS default). The auto option is not available on the Catalyst 6000 IOS; if the speed is set to auto, the duplex follows suit.

TIP

The duplex mode can be autonegotiated only if the port speed is also set to auto (or autonegotiate). Gigabit and 10 Gigabit Ethernet ports can be set to either full- or half-duplex. Beware of a port that has a duplex mismatch, in which one end is full- and the other is half-duplex. This condition can cause poor response and a high error rate. Make sure that both ends of a link are set to autonegotiate or the same duplex setting.

4.

(Optional) Set the port traffic flow control:

COS	[View full width] set port flowcontrol mod/port {receive | send} {off | on | desired}
IOS	[View full width] (interface) flowcontrol {send | receive} {desired | off | on}

A switch port can receive pause frames, causing transmission to stop for a short time while buffers at the far end are full. By default, receive processing is off for all switch port types (except 10 Gigabit Ethernet). A port can also send pause frames if its buffers get full. By default, send is on for Fast Ethernet, desired for Gigabit, and off for all other port types. The desired keyword is available for Gigabit ports only, where autonegotiation is inherent.

5.

(Optional) Control port negotiation:

COS	set port negotiation mod/port {enable |)disable}
IOS	(interface) [no] negotiation auto

By default, link negotiation (flow control, duplex, fault information) is enabled on Gigabit Ethernet ports. To disable negotiation, use the disable or no keyword.

6.

(Optional; Catalyst 6000 only) Enable the port debounce timer:

COS	set port debounce mod/port {enable | disable}
IOS	N/A

By default, the linecards wait 300 milliseconds (10 milliseconds for fiber Gigabit ports) before announcing to the main processor that a port has changed state. This "debounces" the up/down state change so that quick changes do not trigger Spanning Tree Protocol (STP), Port Aggregation Protocol (PAgP), Simple Network Management Protocol (SNMP) traps, and so on. This debounce gives a port the chance to settle down to a stable state. If you find that this period is too short, you can enable an extended port debounce for specific ports. When enabled, the debounce period becomes 3.1 seconds (100 milliseconds for fiber Gigabit ports).

NOTE

When this is enabled, port up/down detection is delayed. The normal STP state progression, along with PAgP negotiation, can make a very long delay before a port can become usable. Use this with caution.

7.

(Optional) Optimize the port as a connection to a single host:

COS	set port host mod/port
IOS	(interface) spanning-tree portfast (interface) switchport mode access (interface) no channel-group

Several options are set for the port: STP PortFast is enabled, trunk mode is disabled, EtherChannel is disabled, and no dot1q tunneling is allowed. This optimizes the link startup time when the port is attached to only one host. Notice that COS switches can accomplish this in one command, whereas IOS switches require several.

8.

(Optional) Use inline power where an IP phone is connected:

COS	set port inlinepower mod/port {off | auto}
IOS	(interface) power inline {auto |)never}

On ports or linecards that can support inline power to IP phones, power is supplied if an IP phone is detected on the port by default (auto). If power should never be supplied to a connected device, choose the off or never keyword. See Chapter 14, "Voice," for more configuration information.

9.

(Optional) Allow large or jumbo frames:

COS	set port jumbo mod/port {enable | disable}
IOS	(interface) mtu bytes

By default, the maximum frame size or maximum transmission unit (MTU) that can be switched is 1548 bytes (COS; default disable) and 1500 bytes (IOS; mtu 1500). Sometimes you might need to switch larger packets to improve performance from server to server. To allow switching of packets up to 9216 bytes, use the enable keyword (COS) or set an MTU size of bytes (IOS; 1500 to 9216 bytes).

TIP

Enabling jumbo frame support allows large frames to be switched. If they also need to be routed, make sure that the MTU on the respective router interfaces is set to the same size. Jumbo frame support is available on an MSFC2 with the mtu interface command, but is not available on the Multilayer Switch Feature Card (MSFC).

10.

(Optional) Automatically reenable ports from the errDisable state.

a. Set the timeout period before ports are automatically reenabled:

COS	set errdisable-timeout interval {interval}
IOS	(global) errdisable recovery {interval interval}

If ports in errDisable are automatically reenabled, the ports remain in the errDisable state for interval (30 to 86400 seconds, default 300 seconds). This command is not available in the Catalyst 2900XL and 3500XL IOS families.

b. Choose the causes that will automatically reenable ports:

COS	set errdisable-timeout {enable | disable} reason
IOS	(global) [no] errdisable recovery cause reason

By default, ports in the errDisable state are not automatically recovered or reenabled. If automatic recovery is desired for an errDisable condition, use the enable keyword (COS) or the errdisable recovery cause command (IOS). The ports will be recovered after the errDisable timeout period has expired. Choose one of the following reasons:

- BPDU Port Guard A bridge protocol data unit (BPDU) is received on a port in the STP PortFast state; use reason bpdu-guard (COS) or bpduguard (IOS).

- UDLD A unidirectional link is detected; use reason udld (COS and IOS).

- STP Root Guard Use reason rootguard (IOS only).

- EtherChannel misconfiguration The EtherChannel ports no longer have consistent configurations; use reason channel-misconfig (COS) or pagp-flap (IOS).

- Trunk negotiation flapping Dynamic Trunking Protocol (DTP) is detecting changes from one trunk encapsulation to another; use reason dtp-flap (IOS only).

- Duplex mismatch A high amount of excessive and late collisions are being detected; use reason duplex-mismatch (COS only).

- Port is going up and down Use reason link-flap (IOS only).

- Some other port problem Problems detected by a switch process not in this list; use reason other (COS only).

- All known errDisable causes Ports are put into errDisable if any problem from this list is detected; use reason all (COS and IOS).

The errdisable recovery cause command is not available in the Catalyst 2900XL and 3500XL IOS families.

11.

Enable or disable the port:

COS	set port enable mod/port -OR- set port disable mod/port
IOS	(interface) shutdown -OR- (interface) no shutdown

By default, a port is enabled (enable or no shutdown). To disable the port, use the disable or shutdown keywords.

Ethernet Example

A 10/100/1000 switch port connects a mail server. The port is set to 100 mbps, full duplex. The port is also tuned for a single host so that there are no port startup delays because of PAgP, STP, or trunk negotiations:

COS	set port name 3/1 Mail server set port speed 3/1 100 set port duplex 3/1 full set port host 3/1 set port enable 3/1
IOS	(interface) description Mail server (interface) speed 100 (interface) duplex full (interface) spanning-tree portfast (interface) switchport mode access (interface) no channel-group (interface) no shutdown

Displaying Information About Layer 2 Interfaces

Table 4-2 lists some switch commands that you can use to display helpful information about Layer 2 interfaces.

Table 4-2. Switch Commands to Display Layer 2 Interface Information

Display Function	Switch OS	Command
Port status	COS	show port [mod | mod/port]
	IOS	(exec) show interfaces [type num]
Port error counters	COS	show port mac [mod | mod/port]
	IOS	[View full width] show interfaces counters [broadcast | errors | {module mod-num} | {trunk [module mod-num]}]
Port MAC address used by the switch	COS	show port mac-address [mod[/port]] -OR- show module mod
	IOS	(exec) show interfaces [type num] -OR- show catalyst6000 chassis-mac-address
Port flow control	COS	show port flowcontrol [mod[/port]]
	IOS	show interfaces [interface [mod]] flowcontrol
Port negotiation	COS	show port negotiation [mod[/port]]
	IOS	N/A
Port debounce	COS	show port debounce [mod |)mod/port]
	IOS	N/A
Port inline power	COS	show port inlinepower [mod[/port]
	IOS	show power inline [interface-id] [actual | configured]
Jumbo frame support	COS	show port jumbo
	IOS	(exec) show interfaces [type num]
errDisable recovery and port status	COS	show errdisable-timeout
	IOS	(exec) show errdisable recovery

On COS switches, you can generate and view reports of utilization, traffic volume, and errors on each port in the switch. These TopN reports can prove useful if you don't have network management applications that can generate statistical reports about the switch ports.

1.

Run a TopN report:

COS	[View full width] show top [N] [metric] [interval interval] [port-type] background
IOS	N/A

The report should include the top N (1 to the maximum number of switch ports; default 20 ports) entries using the metric as a sorting key. Metric can be one of util (utilization; the default), bytes (bytes in/out), pkts (packets in/out), bcst (broadcast packets in/out), mcst (multicast packets in/out), errors (input errors), or overflow (buffer overflows).

The TopN report can sample the port data over an interval (0 or 10 to 999 seconds; default 30 seconds). If interval is 0, the report uses the absolute port counters (as seen with show port or show port mac). In addition, reports can be run for specific types of ports as port-type: all (all port types; the default), eth (all types of Ethernet ports), 10e (10 mbps Ethernet), fe (Fast Ethernet), ge (Gigabit Ethernet), or 10ge (10 Gigabit Ethernet).

The switch prompts when the report has started and ended. Be sure to note the report number that is being generated. With the background keyword, you can issue other switch commands while the report is in progress.

2.

View a stored TopN report:

COS	show top report [report-num]
IOS	N/A

A specific TopN report numbered report-num can be viewed. To see a list of all the stored TopN reports, omit the report number. Reports are stored in switch memory and remain there until the switch is rebooted or loses power. To clear TopN reports from memory, use the clear top [all | report-num] command.